1. Field of the Invention
The present invention relates to an intermediate adapter that is used in a camera system including a plurality of cameras having a same mount structure and interchangeable lenses having different protruding amounts toward an image plane from its mount reference surface and that is mounted between a camera and a interchangeable lens.
2. Description of Related Art
In recent advancements in applying digital technologies to cameras, single-lens reflex digital cameras have been proposed, in which 35 mm silver halide films are replaced with electronic image pickup elements, such as a CCD sensor and a CMOS sensor, while maintaining the uniformity of a mount structure to allow a conventional group of interchangeable lenses to be mounted and used in a single-lens reflex camera system, a lens of which can be replaced. However, since the electronic image pickup elements, such as a CCD sensor and a CMOS sensor, having the same size as the 35 mm silver halide films, are expensive, the image pickup elements (for example, image pickup elements having an APS-C size) smaller than the 35 mm films have been mainly used.
In such a single-lens reflex digital camera using an image pickup element smaller than the 35 mm film, since an image circle is smaller than that of the 35 mm film, a quick return mirror in a camera body can be made small, and also the size of the camera body can be reduced. Further, when the quick return mirror is made small compared to those of the conventional case, back focus of the interchangeable lens can be shortened in comparison with that of the conventional interchangeable lens. Then, the image circle is smaller compared to the conventional interchangeable lenses, thereby achieving small-sized interchangeable lenses.
However, when the interchangeable lens having a short back focus is mounted on a body of the 35 mm film camera or a digital camera having an image circle larger than that of the interchangeable lens, the rear end portion of the interchangeable lens and the quick return mirror may interfere with each other. Further, when an interchangeable lens only for a digital camera having a short back focus (having a small image circle) is arranged to be mounted on the camera body of a conventional camera system, there is a disadvantage in that a light flux does not reach the periphery of the image pickup plane.
Thus, mechanism for preventing the interchangeable lens from being mounted on the camera body in such a way is needed.
In Japanese Patent No. 3217273, a camera system, a camera body and an interchangeable lens are disclosed. In this camera system, a common mount is used and the interchangeable lens having a back focus smaller than a predetermined value is restricted to be mounted thereon. In this camera system and the like, by providing a flange portion inside the mount of the conventional camera body and by arranging a fixed protrusion provided on the interchangeable lens having a short back focus to come into contact with the flange portion, the mounting of the interchangeable lens is prevented.
On the other hand, in the lens-interchangeable camera system in which the interchangeable lens is mounted on the camera body, the connection is usually made with a bayonet mount, and there is an advantage in that the connection can be completed quickly and surely at a small rotation angle. In the case of using the bayonet mount, in order to indicate an inserting angular position (phase) when mounting the interchangeable lens on the camera body, mounting indexes are provided on the camera body and the interchangeable lens. A user can complete the mounting by inserting the interchangeable lens into the camera body in a state in which the respective mounting indexes are matched with each other and by rotating the interchangeable lens in a predetermined rotation direction by a predetermined rotation amount until a stopper or a locking pin is locked.
In addition, as an accessory of the lens-interchangeable camera system, there is known an intermediate adapter having the function of allowing close-up image taking by extending the back focus of an interchangeable lens, or of increasing an image magnification of the interchangeable lens, with the intermediate adapter mounted between the camera body and the interchangeable lens.
In Japanese Patent No. 3210586, an intermediate adapter capable of being used in a camera system including interchangeable lenses having different back focuses described above is disclosed.
However, Japanese Patent No. 3217273 and Japanese Patent No. 3210586 do not disclose how a user can recognize that the camera bodies or interchangeable lenses are associated with which systems to perform connection operation, although the mount shape (structure) is identical and the interchangeable lenses or camera bodies can be associated with different back focuses.
In addition, in the camera system disclosed in Japanese Patent No. 3210586, since an intermediate adapter which can be mounted on an interchangeable lens having a back focus larger than a predetermined value and an intermediate adapter preventing mounting of an interchangeable lens having a back focus smaller than the predetermined value are prepared separately, usable combinations of interchangeable lenses and cameras are limited when a user has only one intermediate adapter.